The present invention relates to an X-ray CT scanner and more particularly to an X-ray CT scanner with a scanner rotation mechanism suited for shortening a scan time by rotating a scanner at high speeds.
The X-ray CT scanner produces a cross-sectional image or tomogram of a subject by radiating a fan-shaped X-ray beam from an X-ray tube onto a subject, detecting X-rays that have penetrated the subject with an X-ray detector arranged at a position opposite the X-ray tube, and image-processing data on the detected X-rays.
The X-ray detector has a group of as many as several hundred detection elements arranged in arc, and is placed opposite the X-ray tube with the subject therebetween to form radially distributed X-ray paths in a number corresponding to that of the detection elements. The X-ray tube and the detector are rotated together around the subject through at least 180 degrees to detect X-rays that have penetrated the subject at intervals of a predetermined angle.
Thanks to advantages this X-ray CT scanner has achieved in recent years, such as capabilities of xe2x80x9cscanning a wide range in a short period of timexe2x80x9d and of xe2x80x9cproducing continuous data in a body axis direction and thereby generating a three-dimensional image,xe2x80x9d a spiral CT performing a helical or spiral scan has found a rapidly growing range of applications.
The spiral CT has enabled a substantial reduction in the time required to perform a three-dimensional CT imaging by continuously rotating the X-ray tube and X-ray detector around a subject while moving a table on which the subject is placed, collecting cross-sectional image data in multiple layers over a wide range and reconstructing the data into an image.
The X-ray CT scanner normally includes a plurality of units: a scanner for rotating the X-ray tube and X-ray detector around a subject to take data on the X-rays that have penetrated the subject; a subject table having a table on which the subject is placed; an image processor for processing the X-ray data collected by the scanner to generate a reconstructed image; a display device for displaying the image reconstructed by the image processor; a keyboard with which to enter various commands; and a system controller for controlling a whole system.
The scanner includes an X-ray tube for radiating X-rays against a subject; a collimator for collimating the X-rays radiated from the X-ray tube into a fan beam; a cooler for cooling the X-ray tube; a high-voltage generator for applying a high voltage to the X-ray tube; a multichannel X-ray detector for detecting X-rays that have penetrated the subject; an amplifier for amplifying a weak electric output of the X-ray detector; a rotary member supporting these devices and having a circular hole in which to position the subject at the center thereof; a frame for rotatably supporting the rotary member; a reduction gear and a motor secured to the frame to rotate the rotary member; and a belt (normally a toothed belt) for coupling the rotary member and an output shaft of the reduction gear.
In a scanner of such a construction, when the motor is started, the rotary power of the motor output shaft is reduced in speed by the reduction gear and conveyed through the belt to the rotary member, which then rotates the X-ray tube and the X-ray detector around a subject to produce X-ray projection data (this is also referred to as imaging or scanning) at intervals of a predetermined angle. The rotary member carrying the X-ray tube and the high-voltage generator, because it is capable of counter-weight mounting, can easily establish a mass balance around a rotating axis. Further, since it does not need to be accelerated to high speeds, the rotary member needs only to be rotated at an almost constant speed. Hence, the motor often employs an induction motor based on an open-loop control.
The conventional X-ray CT scanner usually uses a motor for an actuator that rotates the rotary member by reducing the rotation speed of the motor by the reduction gear and transmitting the rotation through a power transmission means such as a belt to the rotary member.
In addition to the X-ray radiation unit and the X-ray detection unit, the rotary member has a high-voltage generation unit for applying a high voltage to the X-ray radiation unit, a cooling unit for cooling the X-ray radiation unit, and an amplifier unit for amplifying a weak electric output from the X-ray detection unit. These units are rigidly fixed to the center of the rotary member from the outer circumferential side by fixing means such as screws.
In the X-ray CT scanner, the widespread use of the spiral CT has led to a significant improvement on a diagnostic technique as described above. There are also growing demands for imaging dynamically moving internal organs such as heart.
To meet these demands, the rotation speed of the X-ray tube and X-ray detector needs to be increased to shorten the scan time. That is, the rotation speed of the rotary member of the scanner must be raised. While the scan time of 1 second/rotation poses no problem for organs other than heart, the imaging of such moving organs such as heart cannot be realized with the rotation speed of 1 second/rotation but requires a higher scan speed of 0.7 to 0.5 second/rotation or even 0.3 second/rotation.
Driving the rotary member at a high rotating speed less than 0.7 second/rotation by using a conventional scanner rotary drive mechanism described above, however, causes the toothed belt to produce a whizzing sound in excess of 70 dB. Since the X-ray CT scanner is used in an inspection room in a hospital where quietness is required, noise of such a level is offensive to the ear of a subject and an operator. To solve this noise problem and still realize a high speed rotation requires the rotary member to be rotated in a direct drive (DD) mode where the rotary member itself is constructed as a rotor of the motor, rather than being rotated through the reduction gear and belt.
Two DD methods are conceivable.
As with general industrial motors, one method uses a permanent magnet to generate a rotary force in the rotary member, and the other induces a rotating magnetic field around the rotary member and uses an electromotive force induced in the rotary member. In the method using the permanent magnet, however, since the rotary member has at its central part a circular hole about 1,000 mm in diameter through which to pass a subject (a subject insertion opening), if a hollow rotor with a hole about 1,000 mm across is to be made from a permanent magnet, the rotary member increases in size and cost and becomes more difficult to manufacture.
On the other hand, in the method using an induced electromotive force, because rotating fluxes generated around the rotor pass through the hole of the rotary member, if a subject is attached with a pacemaker or an electrocardiograph, these devices are likely to be operated undesirably by the rotating fluxes threading through the hole of the rotary member, which must be avoided.
Further, in either method using a permanent magnet or an induced electromagnetic force, a large amount of electromagnetic noise may leak out and interfere with a signal of the amplifier, which amplifies the weak electric output, resulting in a possible degradation of quality of a finally obtained image. To solve such a problem of electromagnetic noise, a measure should be taken to shield the DD motor including the rotary member, which in turn makes the scanner large, hindering the fast rotation of the rotary member.
On the other hand, shortening the scan time poses another problem.
As the scan time decreases, the rotary member must be rotated at an increased speed. The substantial improvements on the diagnostic technique made possible by the widespread use of the spiral CT scanner require an increase in the number of scanning operations performed, which in turn requires the X-ray radiation unit to have a large capacity.
The large-capacity X-ray radiation unit has an increased size and mass, which naturally increases the size and mass of the cooling unit and the high-voltage generation unit.
Since centrifugal forces acting on the units of the rotary member are proportional to the square of a rotational angular speed, when the rotary member incorporating the units such as X-ray radiation unit with increased sizes and masses is rotated at high speed, it is difficult to secure a sufficient mechanical strength in the conventional construction in which the units are mounted to the rotary member with such fixing means as screws.
For improvement on this problem, JP-A-9-56710 proposes an X-ray CT scanner in which the rotary member is formed like a drum and incorporates the units therein. This construction has a drawback that heat produced by the X-ray radiation unit and the high-voltage generation unit is trapped and builds up in the drum.
Especially when the X-ray radiation unit and the high-voltage generation unit are increased in their capacity to shorten the scan time, the amount of heat produced by these units is huge and the interior of the drum in which the heat is trapped becomes very hot, degrading the performance of the units installed in the drum, making it impossible to produce a cross sectional image with high precision, or shortening the service lives of the units.
The above-described JP-A-9-56710 also describes an X-ray CT scanner in which blade members are provided in the drum to send in air as the drum rotates to dissipate heat from inside the drum. The provision of the blade members in the drum, however, raises a problem of causing whizzing noise during the drum rotation, which may deteriorate the inspection environment and make the subject uneasy.
An object of the present invention is to provide an X-ray CT scanner which can increase the scanner rotation speed to reduce the scan time and thereby enable the scanning of such moving organs as heart, by using a scanner rotating mechanism that can reduce rotating magnetic fluxes threading through an opening formed in a scanner rotary member and electro-magnetic noise and secure a sufficient mechanical strength.
To achieve the above objective, a first aspect of the present invention provides an X-ray CT scanner comprising: an X-ray radiation means for radiating X-rays against a subject; an X-ray detection means arranged at a position opposite the X-ray radiation means with respect to the subject; an opening in which to put the subject; a rotary member at least mounting the X-ray radiation means and the X-ray detection means and rotated around the subject; a rotary drive means for rotating the rotary member; and a frame for supporting the rotary member and a rotary drive means; wherein information on the X-rays that have penetrated the subject and are detected by the X-ray detection means is processed to generate a cross-sectional image of the subject; wherein the rotary drive means has a rotor and a stator, the rotary member is used as the rotor, the rotor is provided with a rotor core and a plurality of conductors connected to the rotor core, and the stator comprises at least one set of stator core and stator winding, the at least one set of stator core and stator winding being adapted to clamp the rotor and arranged at opposing positions; wherein a three-phase AC current is passed through the stator winding to generate a rotating magnetic field to rotate the rotor and therefore the rotary member.
The rotor may comprise short-circuit rings provided on an inner circumference and an outer circumference, respectively, of a rotating axis of the rotor core made from a magnetic metal and a plurality of conductors connected to ends of these short-circuit rings.
The rotor may comprise two short-circuit rings of different diameters provided on almost the same circumferential plane of the rotor core and a plurality of conductors connected to ends of these short-circuit rings.
The rotor core may be formed by laminating silicon steel plates punched with a plurality of slots, the conductors may be installed in the plurality of slots, and the ends of the conductors may be connected to the short-circuit rings.
A second aspect of the present invention provides an X-ray CT scanner comprising: an X-ray radiation means for radiating X-rays against a subject; an X-ray detection means arranged at a position opposite the X-ray radiation means with respect to the subject; an opening in which to put the subject; a rotary member mounting at least the X-ray radiation means and the X-ray detection means and rotated around the subject; a rotary drive means for rotating the rotary member; and a frame for supporting the rotary member and a rotary drive means; wherein information on the X-rays that have penetrated the subject and are detected by the X-ray detection means is processed to generate a cross-sectional image of the subject; wherein the rotary drive means has a rotor and a stator, the rotary member is used as the rotor, the rotor comprises a magnetic metal body and conductors connected to both surfaces of the magnetic metal body, and the stator comprises at least one set of stator core and stator winding, the at least one set of stator core and stator winding being adapted to clamp the rotor and arranged at opposing positions; wherein a three-phase AC current is passed through the stator winding to generate a rotating magnetic field to rotate the rotor and therefore the rotary member.
The rotor core may be formed by fixing plate conductors to both surfaces of the magnetic metal body. When a plurality of stators are provided, they may be arranged at almost equal intervals in the circumferential direction of the rotor.
With the above construction, the rotating magnetic fluxes generated by the stator arranged on the outer circumferential portion or on one surface side of the rotor and by the stator arranged on the inner circumferential portion or on the other surface side of the rotor do not leak out of the two stators and thus can be used for producing a rotation torque of the rotor. This construction does not require a large-diameter permanent magnet, which is difficult to manufacture, and therefore achieves a low-noise, high-speed rotation of the scanner by using a direct drive system, which in turn realizes a high-quality image immune from electromagnetic noise and a reduced scan time and enables the scanning of dynamically moving internal organs such as heart.
A third aspect of the present invention provides an X-ray CT scanner comprising: an X-ray tube for radiating X-rays against a subject; an X-ray detector for detecting X-rays that have penetrated the subject; a plate-like rotary member having the X-ray tube and the X-ray detector mounted thereon at opposing positions with the subject therebetween; a support means for rotatably supporting the rotary member; and a rotary drive means for rotating the rotary member about the subject; wherein a unit mounting means having an accommodating portion and a mounting member erected near the accommodating portion is provided on the rotary member at at least one location, and at least one of units is mounted to the mounting member of the unit mounting means from a center side of the rotary member.
The accommodating portion of the unit mounting means may be formed by recessing or cutting away a part of the rotary member, and the mounting member may be integrally erected from the rotary member almost perpendicular to a unit mounting surface of the rotary member on the outer circumferential side of the accommodating portion.
The mounting member of the unit mounting means may be divided into a long side portion and a short side portion, the accommodating portion may be formed by recessing or cutting away a part of the rotary member, and at a location near the accommodating portion the short side portion of the mounting member may be bent almost perpendicular to a unit mounting surface of the rotary member and the long side portion of the mounting member may be secured to an outer circumferential side end of the short side portion of the mounting member.
With this construction, since there are no components around the rotary member that hinder heat dissipation, heat is not trapped inside the rotary member, thus preventing possible performance degradations or reduced service lives of the units mounted on the rotary member. This allows a highly accurate tomogram to be generated over a long period of time. Further, since noise is not produced even at high-speed rotation of the rotary member, the inspection environment can be prevented from deteriorating or giving uneasiness to a subject.
Since the centrifugal forces acting on the units that are generated when the rotary member is rotated at high speed are carried by the rotary member through the mounting members of the unit mounting means, the X-ray CT scanner can secure a sufficient strength to withstand the centrifugal forces without having to increase the mechanical strength of each unit even when the sizes and masses of the units increase. Further, because the centrifugal forces do not act directly on the fixing means that fixes the units to the mounting members, the fixing means can be prevented from becoming loose or failing due to excess centrifugal forces and the units from coming off.
Further, since the mounting members are erected from the rotary member, the rotary member has an increased section modulus, making it possible to improve the rigidity and mechanical strength of the rotary member without having to increase the plate thickness of the rotary member. Compared with the construction in which the plate thickness of the rotary member is increased for improved rigidity and mechanical strength, this construction can minimize an increase in the mass of the rotary member and therefore reduce the moment of inertia when the rotary member is rotated at high speed. Further, because the rotary drive means for driving the rotary member does not require a large capacity, the equipment as a whole can be made small and less costly and power consumption reduced.
Further, since the accommodating portion of the unit mounting means is formed by recessing or cutting away a part of the rotary member and the mounting member is integrally erected from the rotary member almost perpendicular to the unit mounting surface of the rotary member on the outer circumferential side of the accommodating portion, the rotary member can be formed highly accurately by means of casting and at the same time can reliably support even the units of large masses.
Further, the mounting member of the unit mounting means is divided into a long side portion and a short side portion; the accommodating portion is formed by recessing or cutting away a part of the rotary member; and near the accommodating portion, the short side portion of the mounting member is bent almost perpendicular to the unit mounting surface of the rotary member and the long side portion of the mounting member is secured to the outer circumferential side end of the short side portion of the mounting member. This construction can reduce the weight of the rotary member without lowering the rigidity and mechanical strength of the rotary member. Because simply replacing the long side portion of the mounting member can easily deal with the specification changes of the unit, the unit specification changes can be accomplished much more economically than when the entire rotary member is replaced according to the specification changes.